In financial institutions or enterprises, scanning devices such as an optical image sensor are commonly used to scan sheet medium such as paper, banknote, check and so on, so as to generate image data of the sheet medium, and high-speed machine identification and processing are performed on the image data with information processing equipment such as CPU, thereby achieving informationized and efficient documents processing such as accounting and billing.
In scanning the sheet medium such as paper, banknote, check and so on with the scanning device such as an optical image sensor, since the image sensor is an array formed by jointing hundreds or even thousands of CMOS photosensitive elements made of silicon wafers, each of the photosensitive elements being a pixel and an amplifier being arranged at the location of the pixel, and electrical properties of the multiple photosensitive elements and the multiple amplification circuits are non-uniform, the various photosensitive elements and the various amplification circuits would output different voltages when the image sensor is scanning a piece of paper with uniform color and uniform brightness (i.e., the radiation input is uniform), so that values of image brightness generated in an image processing device are different and then regions with in-uniform brightness are formed in the image, which greatly influences quality of the image and in turn influences a result of the identification of the image.
In a calibration method commonly used nowadays, calibration is performed with some pieces of standard gray scale paper (gray scales of the standard gray scale paper are uniform), i.e., image data of the pieces of standard gray scale paper (the gray scales of the standard gray scale paper are uniform), generated by various photosensitive elements with same lighting parameters, are recorded, linear fitting is performed on actual gray values with the standard gray scale, and a gray scale calibration curve is obtained for each of the photosensitive elements.
In practice, for the pieces of standard gray scale paper, the standard gray scales can not be ensured to be uniform during manufacture, the gray scales may vary during use and storage, and the usage life is not long. In this case, the pieces of standard gray scale paper which are used as the reference for the calibration, are not standard, which leads to differences in calibration results; in addition, the pieces of the gray scale calibration paper can be used for limited times, usually needing to be replaced after being used for three to five times, which leads to a relatively high cost of calibration.